1. Field of the Invention
The invention relates to the field of phase detectors, particularly those used for detecting the phase difference between a voltage controlled oscillator and a serial, digital stream of data.
2. Prior Art
Numerous circuits are well-known for detecting the phase relationship between two signals. In one common application, digital data is serially transmitted over a line without the transmission of a separate clocking signal. In effect, the clocking signal is recovered from the data stream. Typically, a receiver includes a voltage controlled oscillator (VCO) which has an oscillator tuned to the frequency of the data (data rate). The phase of a clocking signal from the VCO is compared with the phase of the data and the difference in phase provides an error signal for adjusting the frequency/phase of the VCO. The clocking signal provided by the VCO is used to recover the data from the data stream.
There are many well-known and widely used phase detecting circuits which provide a signal representing the phase difference between the incoming data stream and the clocking signal. Typically these circuits have or need a frequency response four times greater than that of the data rate. The four canonical data receiving loops (Squaring, Remodulator, Inverse Modulator, and Costas) all have paths through which data must pass which traverse two multipliers. At each multiplication stage the required circuit bandwidth doubles, resulting in an overall fourfold increase in signal bandwidth. (See Phaselock Techniques, Second Edition, by Floyd M. Gardner, Ph. D., published by John Wiley & Sons, pages 218-221). Where data is transmitted at a high rate (for instance, at 300 MHz or above) it is difficult to fabricate an ordinary circuit (i.e., constructed of available IC's in leaded packages soldered to a substrate) which has a frequency response of 1.2 GHz. (The present invention requires a maximum frequency response only twice that of the data.)
One prior art technique used to circumvent the need for this very high frequency response (generally used for data rates above 100 megabits/second) employs a surface acoustic wave (SAW) filter. While these filters provide good performance, they are quite expensive. As will be seen, the present invention provides a high frequency phase detector and method of detection for frequencies generally reserved for SAW filters, however, by employing more ordinary circuits.